Signal controlled variable impedance for suppressing static



y 30, 1940- P. M. HAFFCKE 2,209,330

SIGNAL CONTROLLED VARIABLE IMPEDANCE FOR SUPPRESSING- STATIC Filed Aug. 26, i936 v 5 Sheets-Sheet 1 :El J. 14 13 I j i gym l5 w 2d T AMPLIFIER INVENTOR PHILIP M. HAFFCKE ATTORNEY y 1940- P. M. HAFFCKE 2,209,330

SIGNAL CONTROLLED VARIABLE IMPEDANCE FOR SUPPRESSING STATIC Filed Aug. 26, 1936 5 Sheets-Shet 2 I INVENTOR PHILIP M4 HAFFCKE BY ATTORNEY July 30, 1940. P. M. HAFFCKE SIGNAL CONTROLLED VARIABLE IMPEDANCE FOR SUPPRESSING STATIC Fil ed Aug. 26, 1936 5 Sheets-Sheet '5 lNVEN l'O R PHILIP M. HAFFCKE ATTORNEY July 30, 1940. HAFFCKE 2,209,330

SIGNAL CONTROLLED VARIABLE IMPEDANCE FOR sUPPREssING STATIC Filed Aug. 26, 1936 5 Sheets-Sheet 4 INVENTOR PHILIP M. HAFFCKE .22 BY ATTORNEY July 30, 1940.

P. M. HAFFCKE 2,209,330

SIGNAL CONTROLLED VARIABLE IMPEDANCE FOR SUPPRESSING STATIC Filed Aug. 26, 1956 5 Sheets-She 5 E l E1 75 MM I Q INVENTOR PHILIP M HAFFCKE ATTORNEY Patented July 30, 1940 UNITED STATES SIGNAL CONTROLLED VARIABLE IIHPED-T I ANCE FOR SUPPRESSING STATIC Philip M. Haffcke, Washington, D. o. Application August 26, 1936, Serial N0.-97,990 v 21 Claims. (01. 250-2o) (Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0 G575?) This invention relates to preventing the noise due to static from being transmitted through a heterodyne radio receiving system to the audiov output, and more particularly to controlling, by

: the impulses of static, a variable impedance preferably associated with a local oscillator circuit, though it may be applied to other parts of the system, to prevent theproduction of sound due to the static impulses.

Among the numerous objects of this invention are:

To associate with one or more circuits of an oscillator a variable impedance responsiveto static that will detune the oscillator and .cause -the oscillator to cease tobeat against the in-, coming signal wave for the duration of the static impulse;

To change the frequency-oi the oscillator by means of a static-responsive variableimpedance so that the beatfrequency will not pass the following tuned stages or Will-benot of audible frequency;

To provide a variable impedance operatively related to an oscillator to permitreceived energy of normal working amplitude to pass without affecting the operation of the oscillator but which will, upon receiving energy of greater amplitude, be effective to damp outtheoscillations of the oscillator;

vTo provide a method'of operating a vacuum tube network wherein the excess amplitude of, received energy due to static is applied to render the heterodyning inefiective and thereby silence the device for .the durationof such excess ampli tude.

To control, by the amplitude of received radio frequency energy, a variable impedance operatively associated witha portion of a radio re ceiver circuit whereby abnormal amplitudes will so change the value of said impedance for the duration of the excessive amplitudes that no audible eifect of static surges appears.

, In thedrawings:

Fig. 1 is a circuit diagram showing a variable impedance in the form of a vacuum tube connected across the oscillator circuits to change the values of those circuits;

Fig. 2 illustrates the vacuum tube coupled to constitute an additional load on the oscillator circuit when the tube is operating;

In Fig. 3 the variable impedance tube is connected to both the grid and the plate circuit of the oscillator;

Fig. 4 illustrates the variable impedance connected as a fullwaveshunt across the gridcoil of the oscillator;

Fig'. 5 showsthe variable impedance connected as-a short circuit i orthe coil in series with the cathode of the detector tube associated with the oscillator;

Fig 6 depicts the variable impedance con nected across the input to an amplifier;

Fig. 7 shows the variable impedance in series in the plate supply to the oscillator; I

In Fig; Bthe variable impedance both shunts thegrid-coil of the oscillator and works as a series variable resistance in the plate voltage supply tothe oscillator; p v

In Fig. 9 th'e,variable impedance acts both as a series variable resistance in the plate supply to the oscillator and as a variable impedance across the plate'coil thereof; 7 'Fig. ludis'closes a variable impedance in the form of "a full wave rectifier acting across the output of two tubes inj' push-pull, together with another tube'in' the" plate supply of the'push 151111 tubes. I v 4 I A The present method of pre'venting noises dueto' static disturbances from appearing in the audible output of a radio: receiving set depends upon co-mf pletely silencing the set for the duration of such disturbances, which are'in' general of such short duration that the brief -intervals or silence will not be noticeable. but even'i'n those cases where theyfare apparent the'effect is much less disagreeable and results in smaller interruption of reception than when crashes of static occur. This is "efiecte'd by connecting a signal controlled variable impedance to the ,set in various Ways suitable to effect the desired result.

By way of exposition and explanation, the variable impedance is herein disclosedas a vacuum tube of thehigh mu type'designed for Class B operation, biased to cut off the plate current and thus act as an extremely high impedance across the-circuits ,to' which it is connected; the grid being connected to have amplified and rectified'radio frequency voltage impressed thereon. When the amplitude of the received signal voltage does not go above the normal working value the tube does' not afiect the operation of the set, but when it is sent into the Working part of the Eg-I curve by swinging the grid strongly positive dueto excess signal amplitude the plate impedance-immediately drops to a low value and the audible output of the set is reduced sub-. stantially to zero for. the duration of the excess amplitude.

Itis well known that an oscillating circuit will not cease oscillating instantly, but will persist for several cycles, growing progressively weaker, and also, when first excited, the oscillations will not reach their maximum amplitude instantly but rather build up progressively until the maximum is reached. This lag in building up and dying out is advantageous in my system, as it rounds the otherwise sharp corners of the envelope when the above mentioned tube goes into and out of operation, which eliminates more or less the danger of shocking the following stages into oscillation.

Fig. l is a circuit diagram of my invention as applied to a receiver having the grid ll of first detector tube l2 coupled to a tuned input 13 by a condenser l4 and having the usual grid leak,v

l5. Cathode l6 of tube 12 is in series with a coil i! that is coupled to plate coil l8 and grid coil 19 of oscillator tube 20 whereof the grid 2| is connected to cathode 22 by grid leak 23 and to variable resonance circuit consisting of coil [9 and variable condenser 24 by a condenser 25. Plate 26 of tube 20 is connected to a source of positive potential through coil 18, resistance 21, and condenser 28 that may be shunted by switch Connected to the input of tube [2 by condenser 28 and conductor 29 is a high gain variable transconductance amplifier 30 operating on the principles disclosed in my copending application, Ser. No. 87,404, filed June 26, 1936, wherein the ratio of static to signal is greatly increased by shifting the bias to the steep part of the characteristic curve so that st'atic that would normally appear in the output 31 at perhaps 5 db. greater than the signal output will be raised to an eifective value on the order of 50 db. higher. Coupled to coil 3| is a coil 32 having its terminals connected to anodes 33 of a rectifier tube 34 whereof the cathode 35 is connected by conductor 36' to grids 36 of tube 31 and through impedance 38 to the midpoint of coil 32 and to ground. However, instead of the amplifier 30 ahead of rectifier tube 34, a D. C. amplifier of suitable characteristics may be connected between rectifier 34 and tube 31 as indicated at X. Tube 31 is biased to cutoff at normal signal amplitudes, and functions as a high impedance across the particular circuit of oscillator 20 to which it may be connected.

Tube 37 is a Type 53 or similar tube designed for Class B operation. If a constant bias of two to five volts negative be applied to the grids 36 the impedance across the cathode I38 and anodes 39 will be very high, yet a small positive bias on grids 36 will cause the plate impedance to fall to a few thousand ohms. Plates 39 are connected through radio frequency choke 4i] and variable resistance 41 to the source of positive potential. Plates 39 may be connected by conductor 42, condenser 43 or switch 44, and switch 45 to either the plate circuit or the grid circuit of tube 20, though ordinarily instead of employing switch 55 there will be a permanent connection to one or the other of these circuits, and when tube 31 goes into operation as a low impedance it will act as a very low resistance shunted across coil l8 or coil 19, depending upon the circuit to which tube 31 is connected. The voltage impressed upon plates 39 is suificient to insure a change in impedance adequate to elfect the desired results when tube 3! passes current.

It is apparent that the potential of grids 36 will depend upon the flow of current from rectifier tube 34 through impedance 38 due to voltages set up in coil 32 by fluctuations of output of amplifier 30 in coil 3!, and impedance 38 is of such value this potential is two to five volts lower than that on cathode I38 while the amplitude of the energy through coil 3| is within the normal working range and tube 3] .is blocked, but when that amplitude. becomes excessive, as when the signal is affected by static, the increased fiow through impedance 38 will make grids 36 positive with respect to cathode I38 and tube 31 will pass current with concomitant large decrease in the impedance of tube 31. A by-pass condenser 48 will in some cases improve the operation of the device.

When switch 46 is closed condenser 41 is shunted and has no effect on the plate supply of tube 20, and with switch 44 open the condenser 43 will pass radio frequency currents through tube 31 to ground when the impedance of the tube is lowered by grids 36 being swung positive due to radio frequency energy of amplitude above the normal working range being supplied to rectifier 34, it being understood that switch 45 is closed to make connection with one of the works of the oscilla tor. When switch 46 is open and switch 44 is closed the condenser serves as a by-pass to ground and the supply to plate 26 of oscillator 20 is shunted through radio' frequency choke 40, switch 44 and conductor 42, the switch 45 being closed to the plate circuit. When the impedance of tube 31 is lowered by excessive currents through rectifier 34 the potential applied to plate 26 is decreased and the oscillator is detuned. It is apparent that, depending upon circuit values and applied biasing potentials, the oscillator may be silenced or its frequency so changed that there is no audible output due to the beat frequency being out of the audible range, for the duration of the operation of tube 31' as a low impedance, which condition is co-extensive with the duration of excessive amplitudes of received radio frequency energy.

In the embodiment of my invention illustrated in Fig. 2 the plates 39 of variable impedance tube 31 are respectively connected to the opposite ends of coil 50 that is closely coupled with the coils l8 and IQ of the oscillator network, and thus tube 31 functions as a load across the oscillator. The battery 5|, which may be the common power supply for the receiving set, mayor may not be used, connected through a resistance 52 or without that resistance, to lower the minimum impedance of the tube 31 during the periods of operation of that tube. The condenser 53 provides a by-pass for radio frequency currents. The other elements are the same in structure and function as are the like numbered elements of Fig. 1. Fig. 3 shows one plate 39 of variable impedance tube'3'i connected to the grid circuit of oscillator 23 and the other plate connected to the plate circuit of the oscillator, which gives a double damping effect.

Fig. l discloses the use of the variable impedance tube as a full wave shunt across ff grid coil ii? oscillator 20; which gives full wave. damping in the oscillator grid circuit. Tube 3? is connected in series with condenser 5 which is of several times the capacity of tuning condenser 24 and will therefore offer little im dance to radio frequency currents tending to flow throu h the anode circuits of tube31. If the capacity of condenser 54 be of smaller value than that of tuning condenser 24 it will, when the tube 3 is passing current in response to excessive amplitude, effectively alter the frequency of the oscillator so that the intermediate frequency generated will be other than that for which the intermediate frequency stages of the receiver are tuned, and hence the resulting 1ntermediate frequency will not be passed.

' The variable impedance tube 3'! is shown in Fig. 5 as being connected to the cathdde lead of the mixing tube. The coils I8 and I9 in the oscillator tube circuits should be closely coupled to pick-up coil l1, and the passing of current by tube 3'! will then sufficiently alter the constants of the circuits to prevent audible effects due to static; either by detuning the oscillator or by damping.

Fig. 6 illustrates the application of the variable impedance tube 31 to reduce the output of radio frequency transformer 55, which maybe an interstage transformer used as the input-to amplifier or'detector tube 58. Plates 39 are connected, through condensers 51, across center tapped secondary 58 of transformer 55 and thus acts as a full wave impedance of low value when the tube 37 is passing current, the AC on one half of the cycle going through one of the plates I 39 and the cathode connections and through the other plate 39 on the other half of the cycle, when the grids 36 are swung into the operating portion of the EgIp' curve. Thus, when the condensers 51 are large, tube 31 acts on each 1 half of the wave much as a variable non-inductive resistance of equal value across the tuning circuit 58--59. However, when the condensers 5'Iare small (of the order of .00025 mf.) the action is that of an additional capacity in parallel with condenser 59 across secondary 58. P- tcntiometer 80 permits adjustment of the positive potential applied to plates 39- to secure the proper internal impedance in tube 31, the radio frequency chokes 6| being interposed between plates 39 and the source of positive potential.

In Fig. 7 the variable impedance tube 83 is in series in the oscillator plate supply, with its anode 92 connected to the source of plate supply and its cathode 61 connected to plate coil I8 of the oscillator, the grid coil and the pick-up coil of the oscillator being designated I9 and I1, respectively, as above. When the input to rectifier M through secondary 32 is of normal amplitude the voltage drop caused by the current from rectifier 64 through resistance 65 is small and the bias impressed on grid 68 of tube 63 due to the drop through resistance 65 does not materially afiect the plate current through the tube 63 and coil I8, but when excessive amplitudes are received the relatively large current through resistance 55 impresses on grid 65 a negative potential that will, with propercircuit values, reduce the plate potential on the oscillator to cut-ofi. Condenser 51 and theconnections to ground through condensers G8 and 69 may be used or omitted as operating conditions may indicate.

The embodiment in Fig. 8 employs the variable im edance to attain a dual effect; one, to control the plate supply as in Fig. 'l and two, at the same time either to damp the grid circuit f the oscillator or throw it out of tune. The variable impedance tube I0 has two anodes 'II and I2. respectively connected to grid circuit I3 of he oscillator and in series in the plate suppl. of the oscillator, the latter as i'rTFig. 7. Current through anode II is controlled by grid 74 connected to the high voltage end of resistance 15 in the output of rectifier I6 and anode 12 cooperates with grid 1! connected to the low voltage end of that resistance, cathode 18 being connected to the mid-point of resistance I to adapt it to cooperate with both grids while they function oppositely during surges of static. The several. elements are suitably biased so that While the received energy isof normal amplitude grid l -l substantially cuts off anode II while grid 'I'l permits free fiow of current in the plate sup-'- ply circuit including anode I2 and cathode 18. However, when excessive surges are impressed upon rectifier 16 the grid 14 is, due to the effect of resistance I5, made strongly positive with respect to cathode I8 and current passes from grid circuit I3 of the oscillator into the coil (8 and the oscillator plate circuit, while simultaneously grid l! is swung negative to interrupt the plate supply. Whether condensers I9, 80 and8l are used depends upon operating conditions. Condenser 8| may have a capacity of .001 to .0005 mi. to avoid introducing too great time factor. It is apparent that tube II acts both'as a shunt across the grid coil I9 of the oscillator and as a signal controlled variable resistance in the plate supply of the oscillator.

Fig. 9 is in general similar to Fig. 8, anode 82 controlled by grid 83 acting as a variable resistance in the plate supply of the oscillator wh leanode 84 controlled by grid 85 functions to va y the impedance across plate coil I8. It is believed the operation is apparent from the foredescription of Fig. 7.

While transformers 85 and 81 in Fig. are indicated as being audio frequency transformers, it is to be understood that the embodiment disclosed in this figure may be used with equal success one/radio frequency stage. Tubes 88 and 89 are connected in push-pull to the secondary of transformer 86 and have a common grid leak 90 and condenser 9|. Plate supply for tubes 88 and 89 is through conductor 92 connected to the center tap of the primary of transformer 81, to the opposite ends of which the plates of these tubes are connected. Anodes 93 and 94 of variable impedance tube 95 are respectively connected to the plates of tubes 88 and 89 and thus tube-95 acts as afull wave variable impedance constituting a load of comparatively low resistance across tubes 88 and 89 to absorb the shock of quickly cutting oif the plate current of those tubes when excessive surges are received, thus smoothing the operation. Variable impedance tube 98 is connected with the cathode 91 and anode 98 in series with the source of plate supply and conductor 92 so that it exercises control over the plate supply as above described in connection with the similar hook-up in Fig '7. Radio frequency energy is fed to rectifier 99 through transformer I00 and the current through the rectifier traverses series resistances Illlv and I02. Grid I05 of tube 95 is connected to the high voltage end of resistance IOI grid I05 of tube 96 is connected to the low voltage end of resistance I02 and cathodes I03 and I04 of tubes 95 and 96, respectively, areco n nected between those resistances. The tubes are all so biased that while the energy put into rectifier 99 is Within the normal working range of amplitude tubes 95 and 96 have little effect on the operation of the system of which tubes 88 and 89 are a part, but when excessive surges are im-' pressed upon rectifier 99 the grid I05 is swung positive and tube 95 functions as a low impedance load across tubes 88 and 89 and grid I06 is made strongly negative, which cuts oil or greatly reduces the-plate supply to tubes 88 and 89. When my invention is applied as in Fig. 10 to a-radio frequency stage the circuit parameters must be suited to the frequency involved, the input to the push-pull tubes should be from the same source as the input to the rectifier tube, and there should be no reaction between the controlled circuits and the rectifier that will interfere with the rectifier preamplifier receiving an unaltered signal.

In all the embodiments above described, the various tubes are so biased that the variable impedance tube has little effect upon the operationof the receiving system so long as the received energy is of normal amplitude. If it is found that there is a perceptible reduction of normal output due to the variable impedance tube another stage of amplification can be added to overcome it.

From the foregoing exposition of my invention itis apparent that the signal controlled variable impedance may be applied to the circuits of tubes used as buifers, detectors, amplifiers, etc. The foregoing specific applications of my invention are to be construed as being given by way of explanation and illustration and not by way of limitation, since those skilled in this art will perceive other combinations whereby the principle of a signal controlled variable impedance for reducing the audible output from a radio receiver for'the duration of received impulses of excessive amplitude may be utilized.

1 This invention may be manufactured and used by or for the Government of the United States of America for governmental purposes without payment of royalties thereon or therefor.

I claim:

' "i. In a radio receiving system, a detector tube having an anode, a control grid and a cathode, an oscillator tube having an input and an output network each of which includes a ,coil, said coils being coupled together, a coil in the cathode circuit of said detector also coupled with the said coils, a rectifier tube, means to apply radio frequency energy to control the output of said rectifier tube, a tube connected across a network of said oscillator and normally constituting a high impedance, means connecting the output of said rectifier to control the output of the last mentioned tube whereby the output of said rectifier in response to excess amplitude of radio frequency applied to said rectifier will reduce the impedance of said last mentioned tube to a relatively low value, and voltage divider means connecting the output of the last mentioned tube and of the oscillator to a source of positive potential.

2. In a radio receiving system, a detector tube, an oscillator unit associated. with said detector to modify the output thereof, variable impedance means of normally high value connected to the output of said oscillator, and means responsive to above-normal amplitudes of received radio frequency energy associated with said impedance means to reduce the impedance thereof for the duration of such above-normal amplitudes and thereby damp out the oscillations of said oscillator.

3. In a radio receiving system, a detector tube, an oscillator unit associated with said detector to modify the output thereof, a tube of normally high impedance connected to the output of said oscillator, and means responsive to above-normal amplitudes of received radio frequency energy connected to said last mentioned tube to reduce the impedance thereof for the duration of such above-normal amplitudes and thereby damp out the oscillations of said oscillator.

- 4. In a radio receiving system, a detector tube, an oscillator unit, including a-resonant network,

associated with'said detector to modify the output thereof, a tube of normally high impedance associated with the resonant network of said oscillator unit, and means responsive to above-normal amplitudes of received radio. frequency energy connected to the last mentioned tube to reduce the impedance of said last mentioned tube for the duration of said above-normal amplitudes and thereby detune said resonant network.

5. In a radio receiving system, a detector tube, an oscillatorunit associated with said detector to modify the output'thereof, variable impedance means of normally high value associated with the output of said oscillator, and means responsive to above normal received energy to reduce the impedance of said means andtherebyso change the circuit values of said oscillator unit that oscillation will not continue.

6. In a radio receiving system, means including an oscillator unit for heterodyning an incoming wave, means'r-esponsive to the amplitude of the incoming wave, and means controlled by the last mentioned means to vary the impedance of said unit to damp out the oscillation of said unit, and thereby prevent the production of sound of audible frequency for the duration of Wave amplitudes above a predetermined value.

7. In a radio receiving system, means including an oscillator unit for heterodyning an incoming wave, 'means responsive to the amplitude of the incoming wave, and means controlled by the last mentioned means to vary the impedance of said unit to prevent-the production of beat frequency for the duration of Wave amplitudes above a predetermined value.

8. In a radio receiving system, means including an oscillator unit for heterodyning an incoming Wave, means responsive to amplitudes of the incoming Wave above the desired maximum signalreception value only, and means controlled by the last mentioned means to vary the impedance of said unit to change the beat frequency in the output of said oscillator for the duration of wave amplitudes above a said signal-reception value.

9. In a radio receiving system, a vacuum tube and an operating network immediately associated therewith, a second tube provided with elements to pass current on both halves of a wave and biased to have normally a high impedance having its output connected to said network, means responsive to the amplitude of incoming radio frequency waves when such amplitude is above the maximum desired signal-reception value only, and means controlled by said means and opera tively connected to said second tube to reduce the impedance of said second tube to a low value for the duration of amplitudes of either half of incoming waves above said maximum desired signal-reception value.

10. In a radio receiving system, a vacuum tube and an operating network immediately associated therewith, a second tube provided with elements to pass current. on both halves of a wave and of high amplification factor biased to have normally a high impedance having its output connected to said network, means responsive to the amplitude of incoming radio frequency waves when such amplitude is above a predetermined value only, and means controlled by said means and operatively connected to said second tube to reduce the impedance of said second tube to a low value for the duration of above-normal amplitudes of either half of incoming waves.

11. In a radio receiving system, an oscillator tube, a variable impedance in the anode supply circuit of said oscillator, means responsive to the amplitude of an incoming wave, and means controlled by said means and connected to said variable impedance to raise said impedance to a high value for the duration of above-normal amplitudes of incoming waves.

12. In a radio receiving system, an oscillator tube, a second tube having its output circuit in series in the anode supply circuit of said oscillator, a rectifier tube having its output connected to the grid of said second tube, and means operatively related to said rectifier to impress radio frequency voltages thereon, the biasing on said second tube being such that it normally passes current but upon reception of above-normal voltage amplitudes by said rectifier tube the said second tube is biased to cut-or": for the duration of such amplitudes.

13. In a radio receiving system, a pair of vacuum tubes connected in push-pull, a first variable impedance connected across the outputs of both of said tubes, 3. second variable impedance connected in series in the common anode supply of said tubes, and means responsive to the amplitude of received radio frequency energy to keep the first of said impedances at a high value when said amplitude is in the normal operating range and simultaneously to keep said second i1npedance low but to reverse the values of said impedances while the said amplitude is above a predetermined value.

14. In a radio receiving system, a pair of vacuum tubes connected in push-pull, a variable impedance connected in series in the common anode supply of said tubes, and means responsive to the amplitude of received radio frequency energy to keep said impedance at a low value when said amplitude is within the normal operating range but to raise said impedance to a high value when said amplitude is above a predetermined value.

15. In a radio receiving system, a pair of vacuum tubes connected in push-pull, a first tube having two anodes, a grid and a cathode, the anodes being each respectively connected to the output of one tube of said pair, a second tube having its anode and cathode connected in series in the common plate supply of the tubes of said pair, a rectifier tube, means to impress radio frequency voltages thereon, a series resistance connected to the cathode of said rectifier, means connecting the grid of said first tube to the high voltage end of said resistance, means connecting the grid of said second tube to the low voltage end of said resistance, and means connecting the cathode of said first tube to said resistance at a point intermediate the ends of the resistance but nearer the low voltage end thereof.

16. In a radio receiving system, a pair of vacuum tubes connected in push-pull, a tube having two anodes, a grid and a cathode, the anodes being each respectively connected to the output of one tube of said pair, a rectifier tube, means to impress radio frequency voltages thereon, a resistance in series with the cathode of said rectifier, means connecting the grid of said tube to the high voltage end of said resistance and means connecting the cathode of said tube toa point of lower voltage on said resistance.

17. In a radio receiving system, a pair of vacuum tubes connected in push-pull, an impedance tube having a cathode and an anode connected in series in the common plate supply of the tubes of said pair and also having a grid, a

rectifier tube, means to impress received radio voltages on said rectifier, a resistance in series with the cathode of said rectifier, means connecting the grid of said impedance tube to that end of said resistance that is negative as to the po ,tential drop due to current from said rectifier a variable impedance and so biased as normally to have no appreciable effect on the operation of said system, a rectifier tube fed from said channel and operatively connected to thelaforesaid tube to change the impedance thereof andthereby so alter the circuit values as to prevent audible output for the duration of excessive amplitudes of radio frequency energy impressed on said rectifier tube and means to impress radio frequency energy on said rectifier.

19. A method of eliminating audible effects of static from the output of a radio receiver that includes an oscillator having an associated net work, which comprises increasing the static-tosignal ratio of received radio frequency energy, rectifying theenergy with said increased ratio, and applying said rectified energy as a factor to maintains, high impedance across at least a portion of the said network when the amplitude of said energy is within the normal operating range but to reducesaid impedance to a low value while- 20. A method of eliminating audible effects of static from the output of a radio receiver that includes an oscillator having an associated network, which comprises rectifying received radio frequency energy together with any static impulses affecting the same, and applying said rectified energy as a factor to maintain a high impedance across at least a portion of the said network when the amplitude of said energy is within the normal operating range but to reduce said impedance to a low value while said amplitude is above normal, thereby preventing the production of audible beat frequency for the duration of said excessive amplitude.

21. A method of eliminating from the output of a radio receiver audible effects of static, which comprises rectifying, during periods only when static above a predetermined amplitude is present, received signal-bearing radio frequency energy together with any static impulses aifecting the same, and applying the rectified energy effectively to reduce a supply of energy necessary to transfer the signal carried by said radio fre- 

